There are numerous applications in which it is desirable to indicate when gaseous oxygen concentration departs from--i.e., becomes either greater than or less than--one or more predetermined threshold concentration levels. For example, in the home health care environment, it is desirable to monitor the output of an oxygen concentrator to determine when the output oxygen level decreases below a minimum desired limit, such as eighty-five percent oxygen. In other industrial and commercial applications, it is desirable to maintain oxygen concentration between preset lower and upper limits. Current devices for monitoring oxygen concentration are expensive and subject to tampering at the application site. It is therefore a general object of the present invention to provide a method and apparatus for monitoring concentration of oxygen in a test gas and indicating when such concentration departs from one or more preselected threshold levels, which are inexpensive to manufacture and implement, which function reliably over an extended operating life, and/or which can be selectively reprogrammed at the factory or in the field by properly trained and equipped personnel while resisting tampering by unauthorized or untrained personnel.
Apparatus for monitoring gaseous oxygen concentration in accordance with a presently preferred embodiment of the invention comprises an oxygen sensor for providing an electrical sensor signal that varies as a function of oxygen concentration at the sensor. Processor circuitry compares oxygen concentration indicated by the sensor signal to at least one threshold level, and indicates when such oxygen concentration at the sensor departs from such threshold concentration level. The apparatus is calibrated by exposing the sensor to a calibration gas having an oxygen concentration equal to the desired threshold concentration level, and storing in the processor circuitry electrical indicia indicative of operating characteristics of the sensor at such threshold oxygen concentration level. When the apparatus is thereafter employed for monitoring a gas of undetermined oxygen concentration, the operating characteristics of the sensor reflected by the sensor output signal are compared to the prestored indicia for determining when oxygen concentration at the sensor crosses the threshold concentration level.
In the preferred embodiment of the invention, the processor circuitry is microprocessor-based and may be programmed to detect a plurality of oxygen concentration levels by sequentially exposing the sensor to calibration gas at the various oxygen concentration levels, and storing electrical indicia indicative of operation of the sensor at each such calibration level for later comparison to the sensor output during use. Such calibration operation preferably is performed at the time of manufacture, and the indicia of one or more calibration levels is stored in non-volatile memory for subsequent use in the field. The apparatus circuitry and the sensor are mounted on a circuitboard assembly, with the sensor and cardedge electrical contacts disposed along one edge of the board. The board may be plugged into calibration apparatus in which the contacts are connected to calibration control circuitry and the sensor is exposed to test gas at desired oxygen concentration through a manifold in the calibration apparatus. Recalibration in the field is either not possible, or requires special knowledge and equipment only possessed by a trained technician.